Undesired dust emissions are produced by coal and other bulk materials during their handling, storage, processing and utilization. Such dust emissions are considered to be a nuisance and, in the case of many of these bulk materials, can also constitute a health and safety hazard. As a result, dust from coal and other such bulk materials must be controlled within safe limits, which in many cases have been specified in federal, state or local legislation,
Industrial and utility plants handling and storing coal and other bulk materials have therefore found it necessary to employ a variety of control technologies in order to reduce the potential health and safety hazards posed by such dust emissions, as well as to comply with applicable legislation. These dust control technologies can be separated into the general categories of mechanical and chemical technologies. In the case of mechanical techniques, devices such as fabric filters and cyclones have been used to collect and dispose of such dust emissions. While mechanical controls can be very efficient, they do often require a large capital expenditure, and they are not always applicable to all of the unit operations that can produce dust in a typical handling and storage system.
Chemical controls, on the other hand, employ additives which are generally applied to the material as a liquid spray or foam. The most common such additive for dust control is water, which is generally applied during the unloading, crushing, or conveying operations. Water, while it is useful in suppressing dust at the point of application, quickly evaporates, thus rendering the material dusty again and, in the case of coal, also detracts from its calorific value. Various oils have also been used for dust suppression, but they are not as widely used today, primarily since they are often contaminated with toxic substances and/or because they pose a hazard to local water supplies. In Severn, U.S. Pat. No. 908,041, for example, the use of heavy mineral oil, along with sand belt dust, is disclosed for collecting and holding dust particles.
In the last two decades, a wide variety of chemical agents have been developed for the specific purpose of improving upon the performance of water and oil sprays. The most common additives to water for such purposes have been wetting agents, that is surfactants, which improve the ability of the water to wet and spread onto the particulate material. These surfactants are thus usually added in low concentrations, and can be quite effective in improving the performance of these water sprays on difficult to wet materials, such as coal. To a lesser extent, emulsifying agents and similar substances can also be added to oils to improve their effectiveness.
Foaming agents described, for example, by Salyer et al in U.S. Pat. No. 3,954,662, in this case being aqueous solutions of an interpolymer of a vinyl ester and certain partial ester compounds, have improved upon wetting agent technology, and are now widely used in several bulk material handling systems. These foaming agents are dissolved in water, and compressed or aspirated air is then used to produce a low or high expansion foam for application to the bulk material. The use of such foams permits a reduction in water consumption as compared to conventional spray systems, and is generally regarded as more effective in capturing finer particulate materials. They are, however, considerably more expensive than water or surfactant solutions.
While both water and water solutions of wetting or foaming agents are generally effective in reducing dust at the point of their application, they are unable to control dust emissions from downstream unit operations. This is particularly true during outdoor storage. A utility power plant may, for example, spray the coal with these solutions when it is unloaded from trains in order to reduce dusting during this operation. The coal, however, must then be conveyed to large outdoor stockpiles, where it remains until it is reclaimed for combustion. The length of time in such storage may vary from a few days to several months, and during this period dust emissions are produced by both wind erosion and by the movement of equipment on the piles. In such cases, conventional water sprays or treatments with wetting agents or foams do not persist in performing their described function during such storage, primarily because they evaporate, are absorbed, or are present in concentrations which are far too low to sustain weathering.
As a result, it often becomes necessary to treat such storage piles with coatings or encrusting agents so as to reduce emissions created by wind erosion and the like. For such purposes, latex emulsions are commonly employed as coating agents, as have a wide variety of other substances, such as lignosulfonates, asphalts, waxes, and numerous polymers. However, these agents are only useful as encrusting agents on the surfaces of inactive piles. Once such coatings are disrupted by reclamation operations or vehicular traffic, the effectiveness of such coatings is destroyed.
Sherman, in U.S. Pat. No. 4,383,971, teaches that the surface of a coal stockpile may be protected by covering the pile with straw, and then spraying a coating of asphalt emulsion on top of the straw to prevent it from blowing away. The surface can then be seeded with grass, which will grow to completely cover the surface and provide a high degree of protection. Obviously, however, such a solution is useful only for coal in "dead" storage, where the pile may remain inactive for a period of years. Thus, said pile coatings are of limited usefulness on active piles, where the surface is worked by machinery, etc. On piles with active surfaces, only the sloped perimeters where there is little traffic can be adequately protected by such a coating.
Consequently, a typical bulk material handling facility may employ several different dust control technologies in order to reduce emissions from the combination of various unit operations that make up their handling systems. The cumulative costs of such separate systems for the purpose of reducing dust emissions during unloading, conveying, stack-out to piles, storage, pile reclamation, crushing, etc. can thus be rather excessive.
In order to reduce these substantial dust control costs, so-called "residual" dust suppressants have thus been developed for the purpose of controlling emissions throughout a series of unit operations. These products are typically combinations of solutions of surfactants and foaming agents with other ingredients that can bind the dust particles together even when the moisture evaporates. Salihar, U.S. Pat. No. 4,551,261, describes, for example, a combination of foaming agent and a latex comprising an elastomeric water-miscible polymer. When this material is sprayed as a foam it is effective in reducing dust at the point of application. Furthermore, after the moisture has evaporated, the latex dries to a film which binds particulate together and prevents wind erosion during storage. Another example of such a residual dust suppressant is provided by Kittle, U.S. Pat. No. 4,561,905, which describes an oil in water emulsion applied as a foam. Other residual dust suppressants incorporating lignosulfonates, various polymers, starches, and oils are also sold commercially as residual dust suppressants.
Since the residual dust suppressant is applied to all of the material entering a particular pile, the product can thus continue to reduce emissions, and this can be true even though the pile may be very active. The residual dust suppressants thus combine water spray and coating formulations into products that are effective throughout a series of unit operations.
While the advent of residual dust suppressants has therefore been of assistance in reducing the number and types of dust control systems used in bulk material handling, they are generally quite expensive. The costs of these materials may thus range between 5 and 40 cents per ton treated, depending upon the moisture, size, and composition of the material. Furthermore, the EPA has pointed out that chemical dust suppressants are often by-products or waste materials. Some of these dust suppressants have thus been found to contain heavy metals and PCB's, and the EPA suggests that the user should analyze the dust suppressant for toxic substances, and also test for possible reactivity between the suppressant and the substrate material itself.
It therefore does not appear that ay of this prior art describes the use of cellulosic fibers as a dust suppressant. Strips of Kraft paper have been used to separate layers of coal in piles to prevent oxidation, such as in Brown, U.S. Pat. No. 2,251,321. Furthermore, sawdust has been incorporated into dust absorbing compositions to facilitate floor sweeping, as described in Wolfram, U.S. Pat. No. 892,484. In these cases, however, neither pertains to the problems of dust production during the handling and storage of bulk materials, such as coal.